heating-up corals – third global bleaching announced
U.S. National Oceanic & Atmospheric Administration (NOAA, Coral Reef Watch program) has already predicted a strong El Niño for this year and “as record ocean temperatures cause widespread coral bleaching across Hawaii, NOAA scientists confirm the same stressful conditions are expanding to the Caribbean and may last into the new year”. On October 8th, NOAA together with XL Catlin Seaview Survey, University of Queensland, and Reef Check, therefore “confirmed a global coral bleaching event is underway. The event is expected to impact approximately 38% of the world’s coral reefs by the end of this year and kill over 12,000 square kilometers of reefs” (source: www.globalcoralbleaching.org). Hawaii is currently facing another massive bleaching, with corals there only just recovering from last year's event. American-Samoa is also heavily impacted and warmer-than-usual water temperatures are being recorded at other locations throughout the tropics.
The impact of this global bleaching event may be more severe than the two previous global events (in 1997-1998 and 2010). “Coral reefs are facing collapse if we don’t act on local stressors as well as climate change.” stresses Professor Ove Hoegh-Guldberg (Global Change Institute, University of Queensland), “If this happens, a major part of the biodiversity of the planet, plus ecological goods and services that support hundreds of millions of people will be at risk. There is no time left―we must act.”
photo by XL Catlin Seaview Survey, American-Samoa
What triggers coral bleaching?
“Previous global events in 1998 and 2010 caused widespread and catastrophic coral death at locations throughout the tropics” says Dr. James Guest, coral reef ecologist and a member of the SECORE science board. “Just like any illness in animals, the bleaching response in corals is a symptom of stress and can be triggered by many things. Global-scale bleaching episodes―like the one predicted for 2015/2016― however, are very strongly associated with elevated sea temperatures and stressful levels of sunlight.” What is exacerbating the unusually high sea water temperatures this year is a phenomenon that occurs every few years known as El Niño. Among other things, El Niño events create a band of elevated seawater temperature in central and east-central equatorial Pacific that can subsequently spread further along the tropical coral belt.
Previous global bleaching events occurred during strong El Niños in 1998 and 2010―and this year may get worse. Global warming has already caused rising sea water temperatures. In combination with an El Niño, coral bleaching is more likely to happen.
graphic by NOAA's Coral Reef Watch and XL Catlin Seaview Survey
What happens to corals, when they bleach?
"During bleaching, corals lose their internal micro-algae as a response to environmental stress, in particular higher than usual sea water temperatures (>1°C above the summer maximum). Bleached corals usually appear whitish, because of the lack of their highly pigmented greenish-brownish algae”, explains Prof. Dr. Christian Wild (University of Bremen). This process is reversible, and symbiotic algae may be taken up by the corals again when conditions are more favorable―for instance when seawater temperatures return to normal. However, “bleached corals are often weakened, making them highly vulnerable to diseases from microbial infections. Thus, severe and prolonged thermal stress can lead to large scale mortality of corals, as happened after extensive bleaching events in 1998 in the Indian Ocean, in 2002 in the Pacific, in 2005 in the Atlantic, and in 2010 in parts of Southeast Asia."
During the 2010 global bleaching, affected reefs looked snow-white, creating a bleak atmosphere underwater―once colorful reef sites, now appeared as a sea of white coral skeletons some of them dead and covered with algae. The following photos were taking on the peak of bleaching in the Andaman Sea, Thailand.
Will we lose our coral reefs?
Many animal species depend on the presence of coral reefs, including commercially harvested fish that rely on reefs for shelter and food. The beauty of coral reefs attracts millions of tourists worldwide and tourism provides a major source of income for many tropical countries. A world without coral would not only mean considerably less diversity, but also the loss off billions of dollars in ecological goods and services. Corals are sessile and adults cannot migrate to cooler waters, therefore, recovery of coral populations depends on either regrowth of remaining colonies or settlement of new larvae from the plankton. This recovery process can sometimes take decades.
Is this the ending―do coral reefs still have a chance? Dr. Dirk Petersen (SECORE) ponders: “there is certainly hope that some reefs will survive, since some corals are more tolerant to thermal stress than others. Field observations also show that at least some species may adapt and/or acclimatize to higher temperatures over time. For example, we found a flourishing coral reef that shifted to dominance by reef building brain corals in front of a desalination plant in Curacao. Those corals constantly face elevated temperatures due to the warm waste water and do surprisingly well. There is also evidence that, within a population, single coral individuals may be more resilient to heat stress than others.” Can we utilize the capacity for corals to adapt to thermal stress by assisting evolution in corals? “Seeding reefs with selectively bred super corals that are more thermally tolerant than others may buy us some time, but we are just beginning to understand these processes and we are not ready to apply our technologies at meaningful geographical scales. Even if we can successfully scale up restoration, this only addresses the symptoms rather than the causes of reef decline.” If we manage to rehabilitate coral reefs on a large scale, they will most likely not be the reefs like before, but could still provide crucial ecological and economical services. In the long term, if we do not slow the rate of climate change, there will likely be no hope at all for the future of coral reefs.
photo by Jamie Craggs, Horniman Museum & Gardens
Time to act
The third worldwide global bleaching event is somehow well-timed to meet the 2015 United Nations Climate Change Conference, short COP21, that will be held in Paris, November 30th – December 11th. Participants are the parties of the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol. COP21 has a noble goal―nonetheless an urgently needed one: to establish a legally binding agreement for the world's nations to act on climate change. The objectives are the reductions of greenhouse gas emissions in order to limit global temperature increase to 2C°. With even Pope Francis standing up for action and vigorous admonitions, such as by Sahran Burrow (General Secretary of the International Trade Union Confederation, ITUC) who says “there are no jobs on a dead planet” (Climate Change Conference – Oslo, March 13th, 2015), politicians must act now. Their ultimate goal must be to limit carbon emissions to reach the 2C° threshold of global warming―we should never forget: it's our future.
by Carin Jantzen